This invention relates to a process for the production of binders for coating compositions.
In the manufacture of latices for binders in coating agents, it is desirable to set an optimum size of the latex particles which, in the case of the binders of this invention, means relatively small particles (.about. 1,000 - 2,000 A), since these ensure good film formation ("Farbe und Lack" 74, 561 [ 1968] and 79, 34 [1973]).
The desired particle size can be obtained in a simple manner by adding large amounts of emulsifier at the beginning of the polymerization. However, such a procedure is disadvantageous in two respects. First, such latices have a pronounced foaming tendency during use and second, they are rather unstable in spite of the high emulsifier content, since the surface energy of the particles increases rapidly on account of the reduced particle size.
These disadvantages can be avoided by adding an optimum amount of emulsifier, i.e., the precise quantity necessary to attain the desired particle size. Although such latices have a low foaming tendency, they are insufficiently stable due to the fact that the amount of emulsifier is too low.
A large number of experiments has been conducted to overcome these deficiencies. German Unexamined Laid-Open Application DOS No. 1,795,303 describes a feeding process wherein initially only a portion of the polymerization mixture is employed while the remainder thereof is gradually added during a longer time interval. However, that application acknowledges at page 9 that the process is difficult to handle. Moreover, it is technically expensive, for understandable reasons.
Another possibility for overcoming the above-described difficulties resides in beginning the polymerization with low amounts of emulsifier and adding the remaining quantities of emulsifier, distributed over the residual polymerization period, so gradually that no further appreciable particle formation is incurred which would lead to an undesirably small-particulate latex (DOS No. 1,645,527). However, this process also is technically expensive.
In order to avoid these cumbersome measures, the polymerization has been carried out in the presence of mixtures of anionic and nonionic emulsifiers, since the nonionic emulsifiers, although contributing toward the stabilization, do not affect the particle size to as great an extent. This process has the drawback that particles are formed which are too large in relationship to the amount of emulsifier used, especially if acrylic acid is used as the comonomer.
A further grave disadvantage of the process resides in that such polymerization systems tend toward the formation of gels (IEC 45, 1330 [1953]). Consequently, coatings produced from such latices are technically useless.
The additional use of polyethylene oxides in polymerization recipes is conventional (DOS No. 1,814,209). Latices produced according to the procedure of this reference have a large particle size and can be prepared only with a low solids content. Experiments have shown that latices with a high solids content according to DOS No. 1,814,209 contain very high proportions of coagulate and are unusable.
It is, therefore, an object of the present invention to avoid the disadvantages described above as inherent in the state of the art and to prepare a latex suitable for binders which is of a sufficiently small particle size, of low viscosity, and yet stable. Other objects will be apparent to those skilled in the art to which this invention pertains.